Low coarseness southern softwood pulps

ABSTRACT

Pulp sheets produced from Southern pine fibers are disclosed. Embodiments in which the fibers have a fiber length to coarseness ratio equal to or greater than 0.11, with fiber length (LWAFL) in mm and coarseness in mg/100 m, are found to impart stiffness properties to tissue grade handsheets made therefrom comparable to those achieved using of standard NBSK. Embodiments in which the fibers have a low coarseness (of from about 16 to about 20 mg/100 m, at a fiber length (LWAFL) of from about 1.6 to about 3.0 mm) are found to impart wicking properties to absorbent structures made therefrom that are improved relative to those achieved using standard high coarseness SBSK.

TECHNICAL FIELD

This disclosure relates to pulp and pulp sheets useful for incorporation into absorbent and/or tissue/towel products, and in particular to pulp sheets produced using Southern pine fibers having certain fiber length and fiber coarseness characteristics.

BACKGROUND

The pulp and paper industry in North America produces a large volume of bleached softwood kraft pulp for use in consumer products. Example uses of this kind of pulp include manufacturing disposable tissues/towels (e.g. facial and bath tissue, paper towels, etc.), and disposable absorbent products (e.g. baby diapers, adult incontinence products, etc.), with about eight million tons per year of such pulp used.

Tissue and towel products are usually formed using blends of hardwood and softwood pulps, to achieve desired properties such as softness, stiffness, and durability (e.g., tear strength, tensile strength, etc.). Often these properties exist in a trade-off, in which an increase in one property results in a decrease in another. In addition, the pulp material must be able to withstand the processes used to form the products. Tissue and towel products are generally manufactured by either a wet process or a through-air drying (“TAD”) process. In a wet process, a low consistency suspension (single layer, or multilayer) of a blend of softwood and hardwood pulp fibers (typically in about a 50/50 to 30/70 ratio) is wet laid, dewatered, and pressed, followed by creping and drying, and finished by calendaring and forming the sheets into rolls. In a TAD process, a wet formed sheet is through-air dried, with or without creping, followed by converting.

Although several factors must be considered in choosing an appropriate blend of fibers, longer fiber length is generally preferable to shorter fiber length, and fibers of low coarseness are generally preferable to those of high coarseness. Longer fiber length tends to provide product durability and good machine runnability in the aforementioned processes. Coarseness, generally defined as weight per unit length of fiber, depends on physical fiber attributes including fiber diameter, cell wall thickness, and cell wall density. For example, a high coarseness value usually indicates a thicker fiber wall, giving stiff fibers resistant to collapse. Thin walled fibers, on the other hand, tend to result in flexible fibers and a denser sheet. Such low coarseness features, in tissue and towel applications, yield a better handfeel (e.g., tactile softness) and good tensile strength. The interplay between these two particular fiber properties is important to achieving a pulp that has good suitability for such applications. For example, a good length to coarseness ratio allows high strength development without excessive densification, to maintain high bulk and softness, and also yields high tear strength.

The usual source of softwood kraft pulp fibers for the aforementioned applications is Northern bleached softwood kraft (“NBSK”) pulp, which in North America is produced mainly in the Northwestern US and Canada from a wood chip furnish from softwood trees that typically include lodgepole pine (Pinus contorta), Jack pine (Pinus banksiana), and/or white spruce (namely, Picea engelmannii, Picea glauca, etc.). NBSK, the fibers of which are generally characterized by a fiber length of 2.0-3.0 mm, and a coarseness of 13-18 mg/100 m, yielding a favorable length-to-coarseness ratio of 0.16 or higher, is the pulp of choice to manufacture of tissue/towel products.

With high growth of global tissue/towel consumption, concerns over sustainability and/or constrained supply of old growth boreal forests as well as beetle infestation of many North American forests, many tissue/towel manufacturers have sought effective substitutes for NBSK. Pulp from some bamboo species, with fiber length of about 1.5 mm and a fiber-to-coarseness ratio of about 0.12, has been considered, but has not been widely adopted, for example due to concerns about availability, as well as cost (such as related to adapting manufacturing processes), and so forth. Southern bleached softwood kraft (“SBSK”) pulp fibers, which are generally characterized by high coarseness (e.g., about 20 or higher) and low length-to-coarseness ratio (e.g., around 0.1), have not been considered to be suitable for these applications.

However, the high coarseness of SBSK fibers has been found to be advantageous in disposable absorbent products such as baby diapers. Historically, such products included absorbent cores made entirely from SBSK, at least in part because the higher coarseness fibers were found to be easier to fiberize from roll form, producing lower knot levels and requiring less energy, and providing good capacity for holding body exudates. Since their introduction, superabsorbent polymers (“SAPs”) have been increasingly used in absorbent products, generally in the absorbent core to provide benefits of high liquid holding capacity and a thinner product. However, effective use of SAPs has been determined, to a great extent, by the ability of the pulp in a composite pulp/SAP absorbent core structure to rapidly wick fluid to the SAP. Thus, one additional role of higher coarseness SBSK fibers in absorbent products, in addition to good pad integrity and allowing easier processing, has come to be seen as providing effective wicking.

SUMMARY

Various embodiments of a pulp sheet in accordance with the present disclosure are produced using Southern pine fibers having certain fiber length and fiber coarseness properties. In one aspect an illustrative embodiment can be characterized as a pulp sheet in which Southern pine fibers have a length-to-coarseness ratio equal to or greater than about 0.11 (with fiber length, or more specifically length weighted average fiber length or “LWAFL,” measured in mm, and fiber coarseness measured in mg/100 m). In some variations the length-to-coarseness ratio is at either a coarseness of from about 16 to about 25 mg/100 m and/or a fiber length of from about 1.6 to about 3.0 mm. In some variations the length-to-coarseness ratio is at either a coarseness of from about 19 to about 20 mg/100 m and a fiber length of from about 2.5 to about 2.7 mm.

As described herein, one particular property of some of the aforementioned embodiments is in imparting tensile stiffness to tissue grade handsheets produced therefrom, at a given tensile strength, that is comparable or superior to that provided by use of conventional NBSK. As tensile stiffness is one indicator of softness, the aforementioned embodiments of pulp sheets that include Southern pine fibers may have utility in tissue/towel applications.

In another aspect illustrative embodiments of pulp sheets produced in accordance with the present disclosure can be characterized in that the Southern pine fibers have a coarseness of from about 16 to about 20 mg/100 m and a fiber length (LWAFL) of from about 1.6 to about 3.0 mm, irrespective of the length-to-coarseness ratio.

As described herein, one particular property of some of these embodiments is in imparting superior wicking properties to composite absorbent structures produced therefrom, contrary to expectation.

In yet another aspect an illustrative embodiment can be characterized in that the Southern pine fibers have a coarseness of from about 16 to about 25 mg/100 m and a fiber length (LWAFL) of from about 1.7 to about 3.0 mm.

The aforementioned embodiments may be in roll or bale form. In some embodiments the content of the said Southern pine fibers in the pulp sheet is at least 20%, with the balance being other pulp fibers. In some embodiments the pulp sheet has a basis weight of about 500-900 gsm, and/or a density of about 0.5-0.9 g/cc. In some embodiments, a pulp sheet may include Southern pine fibers having certain fiber length and fiber coarseness properties, such as those listed above, blended with NBSK fibers.

The illustrative properties mentioned above and discussed in greater detail below are not exhaustive. The concepts, features, properties, methods, and other aspects briefly described above are clarified with reference to the accompanying drawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a generalized method of kraft pulping and bleaching appropriate for producing the pulp sheets in accordance with the present disclosure.

FIG. 2 is a representative plot of wicking rates of example absorbent cores made from pulp sheets of Southern pine fibers of varying coarseness produced in accordance with the present disclosure.

FIG. 3 is a representative plot of inclined plane wicking rates of example composite absorbent cores that include SAP and Southern pine fibers of varying coarseness produced in accordance with the present disclosure.

FIG. 4 is a representative plot of pad densities of example composite absorbent cores that include SAP and Southern pine fibers of varying coarseness produced in accordance with the present disclosure, as a function of press pressure in pad formation.

FIG. 5 is a representative plot of pad densities as a function of press pressure similar to that shown in FIG. 4, but with the example composite absorbent cores also including a softening agent.

DETAILED DESCRIPTION

Fiber coarseness, as noted above, is defined as weight per unit length of fiber. Herein, the term “fiber coarseness,” or simply “coarseness,” refers to this measure, expressed as mg per 100 m. This particular expression is sometimes referred to as a decigrex (dg). Coarseness of a fiber sample is determined herein using a FQA Analyzer (OpTest Equipment Inc. Model LDA02 (HiRes FQA), equipped with an auto sampler), which contains a flow cell that uses hydrodynamic focusing to orient curled fibers. A sample is soaked in water before being disintegrated, then formed into a dry sheet on a TAPPI sheet mold, then weighed. The sample is slurried in water to produce an approximately 0.005 wt % suspension. Test aliquots of the dilute suspension of about 20-25 mL are drawn, recorded, and analyzed. Coarseness in mg/100 m (or dg) is calculated from the sample mass and the average fiber length (in mm) determined by the analysis.

Fiber length, also referred to herein as simply “length,” as used herein, refers to the length weighted average fiber length (also referred to as “LWAFL”), as determined by a suitable length measuring device such as the FQA Analyzer according to a process similar to the one described above for measuring coarseness. Length is expressed in mm.

Values for the properties of fiber coarseness and fiber length provided herein are given in terms of the respective units described above, even if such units are not expressly listed. For example, the expression “a coarseness of about 18” is equivalent to “a coarseness of about 18 mg/100 m.” Likewise, values for length-to-coarseness ratios also are given in terms of the respective units described above, even if such units are not expressly listed. For example, in the expression “a length-to-coarseness ratio above about 0.16,” length, or fiber length, is in mm, and coarseness is in mg/100 m.

Wicking rate, or wick rate, refers to the rate at which water wicks upward through an airlaid pad produced from pulp fibers, and is measured by a standardized test method described in detail herein. Wick rate is a characteristic that describes an absorbent property of pulp fiber that is used to assess whether the pulp fiber is a suitable candidate for incorporation into absorbent products. Wicking rate is generally expressed in mm/s.

Inclined plane wicking rate refers to the rate at which water wicks upward through a composite pulp and SAP pad placed at an incline, along the plane of the pad, and is expressed herein as the weight increase of the pad, representing the total amount of liquid wicked, in a specified time period. The value is typically given in grams per time period (e.g., g/500 s). A standardized test method is described in detail herein. One purpose of the test is to determine the amount of simulated urine absorbed over time by an absorbent pad oriented to replicate that of a baby diaper being worn.

Basis weight refers to the dry weight of a material per unit area, and is generally expressed as grams per square meter (g/m² or gsm). It can be measured using a standard such as TAPPI test method T-220. Density refers to the dry weight of a material per unit volume.

Other properties of a pulp sheet, and/or structures made therefrom, such as a handsheet, a tissue grade handsheet, an airlaid pad, a composite absorbent core, and so forth, and testing methods therefor, are described in greater detail below.

As noted above, the pulp sheets of the present disclosure may have utility for incorporation into tissue/towel products and/or absorbent products. “Tissue/towel products” generally refers to various paper products, such as facial tissue, bath tissue, paper towels, napkins, and so forth, and “absorbent products” generally refers to baby diapers and other products adapted to retain body exudates, such as bandages, feminine hygiene products, adult incontinence products, and so forth.

A suitable illustrative method of producing the pulp sheets in accordance with the present disclosure is shown at 10 in FIG. 1. The illustrative method may be thought of as including two broad processing areas, pulping, generally indicated at 12, and bleaching, generally indicated at 14. The pulping process 12 is shown as a kraft pulping process. However, other chemical pulping processes, such as kraft/AQ pulping, soda/AQ pulping, or sulfite pulping, may be used.

In block 100, wood chips are loaded or fed into a digester. The wood chips used in producing the pulp sheets herein may be obtained from Southern softwood, a designation used to describe a group of species of coniferous trees in the genus Pinus that are native to and/or are characteristically grown in Southern regions of the United States. In particular, this group of species includes slash pine (Pinus elliottii), longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), loblolly pine (Pinus taeda), Virginia pine (Pinus virginiana), eastern white pine (Pinus strobus), pitch pine (Pinus rigida), sand pine (Pinus clausa), pond pine (Pinus serotina), Table Mountain pine (Pinus pungens), Monterey pine (Pinus radiata), and bishop pine (Pinus muricata). The term “Southern pine” is often used to refer to Southern softwood trees; herein, the terms are synonymous. Accordingly, “SBSK” refers to bleached pulp that is produced by the kraft process from Southern softwood chips. In general, Southern pine fibers suitable for pulp sheets in accordance with the present disclosure are those having a coarseness of from about 16 to about 25 mg/100 m and/or a fiber length of from about 1.6 to about 3.0 mm. In more particular embodiments, such as those described herein as having particular suitability for tissue/towel applications and/or absorbent applications, Southern pine fibers having a coarseness of from about 18 to about 20 mg/100 m, and/or a length-to-coarseness ratio greater than about 0.11, are appropriate.

Although SBSK fibers are generally thought of as having high coarseness relative to NBSK fibers, the terms “low coarseness SBSK” or “low coarseness Southern pine” herein refer to the lower portion of the coarseness ranges typically found in SBSK and Southern pine fibers, respectively. For example, in one aspect of the present disclosure, low coarseness SBSK is found to impart unexpectedly superior wicking properties to an absorbent structure made with such SBSK, relative to the industry standard, an absorbent structure made with high coarseness SBSK.

SBSK fibers are generally thought of as having a lower length-to-coarseness ratio relative to NBSK fibers. As noted above, NBSK fibers make up the pulp of choice for tissue/towel applications. However, in another aspect of the present disclosure, SBSK fibers having a comparatively lower length-to-coarseness ratio are unexpectedly found to impart tensile stiffness values to tissue handsheets incorporating such fibers that are comparable to values achieved by use of NBSK fibers.

Coarseness of fibers, as well as fiber length, tends to vary with tree age and also with locations within a tree. Accordingly, representative sources of suitable fibers may be those derived from thinnings, portions of trees of a certain age, whole trees of a certain age, and so forth. For example, managed plantation forestry may make trees of younger ages available, such as for conversion to pulp, as stock is periodically thinned out to allow selected trees to mature for eventual harvesting for lumber.

Fiber length and coarseness variation will further vary from species to species. Table 1 is an illustrative listing of fiber length and coarseness for pulp sheets produced from Southern pine fibers in accordance with the present disclosure, specifically SBSK fibers, obtained from loblolly pine (Pinus taeda) trees of various ages, using an ODED bleaching sequence for target brightness of 90.

TABLE 1 Length to Fiber coarseness Description Portion of tree Coarseness Length ratio  6 year bottom 50% 14.6 1.30 0.089 top 50% 13.3 1.32 0.099  8 year bottom 50% 16.1 1.63 0.101 middle 25% 14.7 1.59 0.108 top 25% 14.4 1.67 0.116 10 year bottom 50% 15.8 1.62 0.103 middle 25% 15.5 1.89 0.122 top 25% 14.8 1.67 0.113 10 year whole tree 16.8 1.86 0.111 12 year bottom 50% 19.1 1.97 0.103 middle 25% 18.9 2.13 0.113 top 25% 17.1 1.95 0.114 12 year whole tree 17.8 2.06 0.116 12 year, earlywood* 18 2.5 0.14 13 year, without tops 18.7 1.97 0.105 13 year, with tops 18.8 2.06 0.110 15 year whole tree 22.2 2.20 0.100 Saw mill residuals** 25.2 2.99 0.119 Topwood*** 19.3 2.53 0.131 *earlywood refers to the part of the wood in a growth ring of a tree that is produced earlier in the growing season. **wood chips from slab wood after extraction of lumber from a grade log of a mature (e.g., 25 year) tree. ***the volume of wood that exists between minimum top diameter for a fiber log and minimum top diameter for grade logs.

Example test embodiments used selected loblolly pine wood supplied as logs that were subsequently debarked (if necessary) and chipped using typical mill debarkers and disc chippers, or as chips produced by conventional chipping methods. The chips were blended, then screened over a Black Clawson gyratory screen selected to yield accepts of a desired size. Oversized chips and fines were discarded, and the accepts were blended again and sub-sampled for moisture determination and specific gravity. Charges for the digester were made using 4.00 kg of O.D. chips. For smaller batches, chips were screened over a Williams screen to yield accepts of a suitable size range for baskets used for batch cooks.

Digesters for use herein can include any digester suitable to pulp Southern pine wood. One example of a suitable digester is a continuous digester often referred to as a “Kamyr” digester, so- called after the now-defunct company that designed and built continuous digesters, currently manufactured by companies such as Kvaerner. Such digesters have been used in the pulp and paper industry for several decades, with modifications over the years to improve operation. The digester system may be either a single vessel or a two-vessel system. Kamyr digesters are typically used in kraft or alkaline wood pulping, but may also be used in semi-chemical pulping methods. Other continuous digesters, such as M&D and Pandia digesters, are also suitable for use. However, the pulp sheets may be produced using any batch or other continuous digester.

Referring to FIG. 1, within the pulping process 12, there are several operations, depicted schematically as blocks 100-116. Loading, or feeding chips as discussed above, occurs at 100. The wood chips may be pre-steamed prior to cooking, at 102. Steam at atmospheric pressure preheats the chips and drives off air so that liquor penetration will be enhanced. After the pre-steaming operation is completed, cooking liquor, referred to as white liquor, containing the pulping chemicals may be added to the chips, at 104. The white liquor and chips are then fed into the digester. In kraft pulping, the active chemical compounds are NaOH and Na₂S. Other chemicals may be added to influence or impart desirable effects on the pulping process, as known to those of skill in the art.

Impregnation, at 106, is the period during which the chemicals are allowed to impregnate the wood material. Good liquor penetration helps assure a uniform cooking of the chips.

Cooking, in which lignin and hemicellulose degrade into fragments soluble in the cooking liquor, occurs at 108 and 110. The co-current liquid contact operation, at 108, is followed by the counter-current liquid contact operation, at 110. Cooking of the wood material occurs during these two operations, and yields brown stock. In either, the cooking liquor and chips can be brought to a desired temperature.

Upon completion of the cook operation, the digester contents are blown, at 112. Digester blowing involves releasing the wood chips and liquor at atmospheric pressure, which generally occurs with a sufficient amount of force to cause fiber separation. If desired, the blow tank may be equipped with heat recovery equipment to reduce operating expenses.

At 114, the pulp is sent from the blow tank to external brown stock pulp washers. The separation of black liquor from the pulp occurs at the brown stock washers.

Various process parameters in the aforementioned operations may be adjusted as suitable for the wood chips, such as impregnation time, percent alkali and sulfidity, liquor ratio, initial, final, and interim temperatures, pH values, and so forth. Many desired pulp properties are sensitive to variations of these and other pulping parameters, such as fiber morphology (e.g., kinks, curls, fiber length), pulp viscosity (which relates, for example, to strength and softness of downstream products that incorporate the pulp), kappa number (which relates to lignin content), and productivity or throughput, as desired. Thus, although any suitable pulping method can be used, the particular pulping method employed must be carefully calibrated to preserve and/or achieve these and other desired pulp and fiber properties.

For example, although not specifically represented in FIG. 1, some kraft pulping test embodiments were carried out in a batch digester having a capacity of about 1 ft³, with liquor recirculation ability, using a target kappa number of 25. For even smaller batches, the digester was fitted with smaller baskets, each having a capacity of about 150 g of O.D. chips, for cooking. The liquor to wood ratio used in the batch digester was 6:1, effective alkali was 19%, and at a sulfidity of 31%. The target temperature of 170° C. took approximately 60 to minutes reach, and was maintained for 102 minutes with an H factor of 1700. After pulping the contents were discharged to a slush maker and the excess liquor was drained. Screening was done over an Appleton screen (10 slots). The rejects were collected and weighed.

Chemical pulps, such as those from the kraft process or sulfite pulping, contain much less lignin than mechanical pulps. Bleaching is performed on chemical pulps generally to remove the residual lignin, and thus the process is often referred to as delignification. Following the pulping process 12, the brown stock pulp made from the Southern pine wood chips is bleached, at 14. In addition to lignin removal, bleaching of chemical pulps results in a decrease in the pulp fiber length and viscosity, but does not involve a substantial reduction to the hemicellulose content of the pulp. Bleaching also tends to increase the brightness of the pulp, and thus a target brightness is often used as a reference to determine the extent to which a pulp has been bleached.

Delignification is rarely a single step process and is frequently composed of several discrete steps, or bleaching stages. The bleaching stages are often indicated as a sequence of a series of letters that each represent the chemical or process used in a bleaching stage (e.g., “C” represents chlorine, “D” represents chlorine dioxide, “E” represents extraction with sodium hydroxide, and so forth). Each bleaching stage is generally carried out in a separate bleaching vessel or tower of conventional design. Bleaching sequences that do not use elemental chlorine, for example to avoid byproducts such as dioxins and dioxin-like compounds, are often referred to as “ECE” (for “elemental chlorine free”) sequences. One representative ECF bleaching sequence suitable for bleaching the brown stock pulp made according to the present disclosure is an ODE_(p)D sequence, with the sequence of bleaching stages indicated in 14 as operations 116, 118, 120, and 122. Examples of various bleaching sequences are described in U.S. Pat. Nos. 6,331,354 and 6,550,350, among others. These references are incorporated herein in their entireties.

The first stage of bleaching is an 0 stage, at 116. The 0 stage involves bleaching with oxygen. Oxygen bleaching is sometimes considered to be an extension of the pulping process, but for the sake of clarity this description treats oxygen bleaching as a first stage in a bleaching sequence. Oxygen bleaching is the delignification of pulps using oxygen under pressure. The oxygen is considered to be less specific for the removal of lignin than chlorine compounds. Oxygen bleaching takes place in an oxygen reactor, suitable examples of which are described in U.S. Pat. Nos. 4,295,925; 4,295,926; 4,298,426; and 4,295,927, fully incorporated herein by reference in their entirety. The reactor can operate at a high consistency (consistency of the feedstream to the reactor is greater than 20%) or medium consistency (feedstream consistency ranges between 8% up to 20%). If a high consistency oxygen reactor is used, the oxygen pressure can reach the maximum pressure rating for the reactor, but more preferably is greater than 0 to about 85 psig. In medium consistency reactors, the oxygen can be present in an amount ranging from greater than 0 to about 100 pounds per ton of the pulp, but is more preferably about 50 to about 80 pounds per ton of pulp. The temperature of the 0 stage ranges from about 100° C. to about 140° C.

A D stage, which involves bleaching the pulp coming from the oxygen reactor with chlorine dioxide, is shown at 118. Chlorine dioxide is more selective than oxygen for removing lignin. The amount of chlorine dioxide used in this stage ranges from about 20 to about 30 lb/ton. The temperature of the D stage ranges from about 50° C. to about 85° C.

An E_(p) stage, which involves hydrogen peroxide reinforced extraction with sodium hydroxide, is shown at 120. In this stage, lignin is removed from the pulp using caustic in an amount ranging from about 20 to about 50 lb/ton. The amount of hydrogen peroxide ranges from about 20 to about 60 lb/ton. The temperature of the E_(p) stage ranges from about 75 to about 95° C.

A second D stage, at 122, follows the E_(p) stage, in which the amount of chlorine dioxide used ranges from about 10 to about 30 lb/ton. The temperature of the second D stage ranges from about 60° C. to about 90° C.

A variety of bleaching sequences may be used in processes suitable for making the pulp sheets in accordance with the present disclosure, with the process parameters adjusted as needed. For example, in some of the test embodiments described above, the screened, unbleached pulp was subjected to an ODE_(p)D sequence, starting with oxygen delignification for which 1200 g of O.D. brown stock at 5.5% consistency containing 180 ml of NaOH (5N) was digested at about 90° C. for 80 minutes. O₂ pressure was 100 psi at temperature for 80 minutes. The washed O₂ delignified pulp (kappa number ˜13) was placed in plastic bags, each bag containing 500 g of O.D. pulp. ClO₂ addition level at the first D stage was 1.4% and was done at 65° C. for 45 minutes. The E_(p) stage was done with batches of 1200 g pulp at 5.5% consistency, with 108 ml NaOH (5N) and 51 ml H₂O₂ (3%) added. The material was subjected to O₂ and brought to 100 psi at about 80° C. over about 20 minutes, after which the O₂ was stopped and the pressure held for 20 minutes, then relieved and temperature held for 50 additional minutes. The pulp was removed from the digester, washed, and centrifuged. The final D stage was done in bags, each bag containing 500 g of material, and to it was added ClO₂ (0.5%) and alkali (0.25% NaOH) to reach a final pH <3. The bags were held in a hot water bath for 3 hrs at about 80° C. After bleaching, the pulps were thoroughly washed and centrifuged to a solids content of about 30%.

After a bleaching process such as described above and/or schematically shown at 14, the bleached stock is typically formed into a sheet by depositing a pulp slurry onto a machine wire, followed by dewatering and drying. The dried pulp is then rolled, cut, and/or baled, or otherwise prepared for shipping. The particular form is often determined by factors such as shipping distance or method, and/or downstream use of the pulp. For example, tissue/towel manufacturing processes generally include machinery adapted to use bales of pulp.

The pulp sheets produced in accordance with the present disclosure may have any desired basis weight and density; however, improved operational efficiency is generally achieved within a basis weight range of about 500-900 gsm and/or a density range of about 0.5-0.9 g/cc.

Pulp sheets can be made from a blend of one or more pulps by mixing the pulps in the slurry, typically in a blend chest, then diluted and formed as above. Optionally, the chip furnish for pulp sheets can be composed of a mixture of chips selected from different sources prior to pulping. As an illustrative example, one test embodiment of a 100% Southern pine pulp sheet consisted of a 50:50 mixture of loblolly pine material sourced from sawmill residuals and 11-yr whole tree. The pulp sheets of the present disclosure include at least about 20% of the Southern pine fibers described herein, with the balance being any other desired pulp, such as other SBSK or other Southern softwood pulps, NBSK or other Northern softwood pulps, hardwood pulps, and so forth. Of course, pulp sheets may include less than 20% of the Southern pine fibers described herein, but the beneficial effects provided by the Southern pine fibers as discussed herein have been found to be diluted at such concentrations. The Southern pine fibers used in a pulp sheet may be from one species of Southern pine tree, or two or more, or several species, in a blend. In one embodiment, a majority of the Southern pine fibers are loblolly pine (Pinus taeda) fibers. Further, as noted above, the Southern pine fibers used in a pulp sheet may be from one or more portions of the same or different Southern pine trees, with the trees being the same or different ages, and so forth.

Strength properties of pulp, and tissues made from pulp, may include tensile breaking properties of a handsheet such as tensile strength, stretch or elongation, tensile energy absorption, and tensile stiffness. Tensile stiffness is one indicator of softness of a tissue or towel product.

For testing properties of tissue, tissue grade handsheets were prepared according to TAPPI method T-205, “Forming handsheets for physical tests of pulp,” except using a target basis weight of 20 gsm, and eliminating the “pressing” step (TAPPI T-205 at 7.4). The handsheets are clamped into jaws of a tensile testing instrument (Instron 4422 R), which measures the force at a constant rate of elongation to measure various tensile breaking properties. Tensile strength, tensile index, and tensile stiffness were determined according to TAPPI test method T-494, “Tensile properties of paper and paperboard (using constant rate of elongation apparatus),” modified to use a test span of 100 mm. Tensile strength refers to the maximum tensile force developed in a specimen before rupture, expressed as force per unit width of the specimen, and indicates the potential resistance to direct stress such as during use, and also stress during manufacturing operations. Tensile index (“TI”) is the tensile strength in N/m divided by the basis weight of the specimen. Tensile stiffness is the ratio of tensile force per unit width to tensile strain within the elastic region of the tensile-strain relationship, as defined in TAPPI T-494, and provides an indication of the response of the sheet to converting forces. It is also one indicator of the softness of the sheet.

Table 2 lists properties of example tissue grade sheets prepared from two illustrative SBSK samples of the specified length (LWAFL) and coarseness prepared in accordance with the present disclosure, blended with eucalyptus pulp (available from Fibria Cellulose) in a 1:2 ratio of SBSK to eucalyptus. These example tissue grade sheets are compared with a tissue grade sheet prepared using a blend of NBSK (LL19, available from Terrace Bay Pulp Mill) of the specified LWAFL and coarseness, also 1:2 with eucalyptus, which is representative of a conventional blend used for tissue/towel products. A commercial tissue is also included in Table 2. Tensile stiffness is reported as measured, and at tensile index normalized to 1.

TABLE 2 Tensile Stiffness (MOE) LWAFL/coarseness Tensile at normalized Sample (ratio) Index measured TI = 1 SBSK-1 2.38/21.5 6.88 0.295 0.043 (0.110) SBSK-2 2.02/16.8 6.70 0.219 0.033 (0.120) NBSK 2.25/13.5 8.20 0.332 0.040 (0.167) Com'l tissue — 4.79* — — *GMT (geometric mean tensile strength) determined by the square root of the product of the machine direction tensile strength (“MD”) and the cross-machine direction tensile strength (“CD”)

If the normalized tensile stiffness value of the conventional NBSK blend is considered as a target ceiling value, the example data in Table 2 indicates that a blend using SBSK having a length to coarseness ratio greater than about 0.110 achieves a tensile stiffness value comparable to or lower than that of the NBSK blend. Considering that NBSK is the pulp of choice for tissue/towel applications due to its comparatively lower coarseness and higher fiber length, the findings that comparatively higher coarseness Southern pine fibers impart comparable (or better, i.e. lower) tensile stiffness is not expected. Moreover, the findings that such tensile stiffness values are achieved by lower length-to-coarseness ratio Southern pine fibers is also unexpected, considering that NBSK is characterized by comparatively higher length-to-coarseness ratio. Although not bound by theory, it is thought that blends using SBSK produced in accordance with one aspect of the present disclosure, that is, having a length to coarseness ratio greater than about 0.11, achieves a tissue hand sheet softness—at a given strength—that is comparable to or better than that using conventional NBSK blends.

Several example tissue grade sheets were made from SBSK samples prepared in accordance with the present disclosure, refined to 500 PFI revolutions, and blended with eucalyptus in a 1:2 ratio. The tissue grade sheets were made as noted above, tested for tensile properties, and compared to a conventional NBSK/eucalyptus blend in which the NBSK was refined to the same extent.

Like Table 2, Table 3 lists tensile properties of the tissue grade sheets. Again, tensile stiffness is reported as measured, and at tensile index normalized to 1.

TABLE 3 Tensile Stiffness (MOE) LWAFL/coarseness Tensile at normalized Sample (ratio) Index measured TI = 1 NBSK-r 2.25/13.5 12.1 0.453 0.037 (0.167) SBSK-r1 2.53/19.3 8.2 0.291 0.035 (0.131) SBSK-r2 1.92/18.6 11.1 0.478 0.043 (0.103) SBSK-r3 2.99/25.2 9.6 0.361 0.037 (0.118) SBSK-r4* 2.80/24.0 9.1 0.263 0.029 (0.116) SBSK-r5* 2.46/22.0 11.2 0.361 0.032 (0.118) *Mixture of SBSK samples. LWAFL and coarseness values calculated using rule of mixtures.

Again, a range of length-to-coarseness ratios for the SBSK made in accordance with the present disclosure most suitable to exhibit the desired tissue hand sheet stiffness is greater than about in theory, any upper limit would be that suggested by the LWAFL/coarseness ratios found in Southern pine fibers when pulped in accordance with the present disclosure. For example, SBSK samples produced from loblolly pine (Pinus taeda) trees of various ages were found to have LWAFL/coarseness ratios up to about 0.14. Especially suitable ratio ranges may be determined by the desired tissue properties. For example, LWAFL/coarseness ratios lower than about 0.12 appeared to correlate well to the lowest tensile stiffness values. However, higher LWAFL/coarseness ratios (e.g., around 0.13) were found to correlate well to other properties such as tear strength, bulk, and so forth. Optionally, although it was found that the aforementioned ratio range imparted suitable stiffness characteristics over the available length and coarseness ranges, a preferred coarseness range for some embodiments is from about 16 to about 25 mg/100 m, and a preferred length range for some embodiments is from about 1.6 to about 3.0 mm. A length and coarseness range that imparted particularly favorable tensile stiffness properties for some embodiments was found at a coarseness of from about 19 to about 20 mg/100 m and a fiber length of from about 2.5 to about 2.7 mm.

Additionally, it was found that the desired stiffness values were exhibited by tissue hand sheets produced from blends containing greater than about 20% of the SBSK fibers having a length-to-coarseness ratio within the range. As noted above, the tissue hand sheets (and other tissue products) may be produced from blending the desired proportion of SBSK fibers (e.g., from a pulp sheet of the Southern pine fibers) with other fibers in the formation of the tissue item, or may be produced from a pulp sheet that itself is a blend of SBSK fibers with other fibers (e.g., a pulp sheet wherein the content of the Southern pine fibers is at least 20%).

In addition to applicability in tissue and towel products, the pulp sheets produced from Southern pine fibers in accordance with the present disclosure may also have applicability in absorbent products, such as baby diapers, adult incontinence products, feminine hygiene products, bandages, and so forth. As noted above, higher coarseness SBSK fibers (relative to NBSK fibers) used in absorbent products, and in particular when used in a composite pulp/SAP absorbent core in absorbent products, is conventionally seen as providing effective wicking. In other words, the expectation is that better wicking in such applications will result from higher coarseness fibers. Higher coarseness fibers generally provide increased void space and bulk, as compared to lower coarseness fibers, and the aforementioned expectation is based on the long-standing theory that void space and bulk in a fibrous structure is necessary for effective fluid transfer, with or without SAP in the fibrous structure. However, although it was found that Southern pine fibers in accordance with the present disclosure yields better wicking at higher coarseness in the absence of SAP, it was unexpectedly found that such fibers, in the presence of a large proportion of SAP, yield better wicking at lower coarseness.

The ability of a material to wick fluid, as noted above, can be measured in several ways. Wicking rate refers to the rate at which water wicks upward through an airlaid pad produced from pulp fibers. Inclined plane wicking rate refers to the ability of water to wick upward through a composite pad made from pulp fibers and SAP that is placed at an incline, along the plane of the pad.

Wicking rate is determined herein using the Automatic Fiber Absorption Quality (AFAQ) Analyzer (Weyerhaeuser Co., Federal Way, Wash.), according to the following procedure. A dry 4-gram sample of the pulp composition is fluffed in a hammermill, then passed through a pinmill and airlaid into a tube. The tube is then placed in the AFAQ Analyzer. A plunger then descends on the airlaid fluff pad at a pressure of 0.6 kPa. The pad height is measured, and the pad bulk (or volume occupied by the sample) is determined from the pad height. The weight is increased to achieve a pressure of 2.5 kPa and the bulk recalculated. The result is two bulk measurements on the dry fluff pulp at two different pressures. While the dry fluff pulp is still compressed at the higher pressure, water is introduced into the bottom of the tube (to the bottom of the pad), and the time required for water to wick upward through the pad and reach the plunger is measured. From this, the wick time and wick rate are determined. The bulk of the wet pad at 2.5 kPa is also calculated. The plunger is then withdrawn from the tube and the wet pad is allowed to expand for 60 seconds. In general, the more resilient the sample, the more it will expand to reach its wet rest state. Once expanded, this resiliency is measured by reapplying the plunger to the wet pad at 0.6 kPa and determining the bulk. The final bulk of the wet pad at 0.6 kPa is considered to be the “wet bulk at 0.6 kPa” (in cm3/g, indicating volume occupied by the wet pad, per weight of the wet pad, under the 0.6 kPa plunger load) of the pulp composition. When the term “wet bulk” is used herein, it refers to “wet bulk at 0.6 kPa” as determined according to this procedure. Absorbent capacity is calculated by weighing the wet pad after water is drained from the equipment and reported as grams water per gram dry pulp.

Inclined plane wicking rate is determined herein using an apparatus that includes a 30-degree incline frame with a balance link upon which the test pad is placed, held in place with support pins and/or clamps. The balance is tared, and the bottom edge of the pad is brought into contact with the surface of a reservoir charged with liquid, generally a 0.9% saline solution to simulate urine. A timing program is started, which measures and records liquid uptake in grams at 5-second increments. The test is generally allowed to run for approximately 15 minutes, with the inclined plane wicking rate reported as the weight increase of the pad, attributable to the amount of liquid wicked, in 500 seconds. Sample preparation for the inclined plane wicking test was done using SAP (BASF 9400) and fluffed pulp at a 30:70 pulp to SAP ratio, formed on a laboratory pad former at a basis weight of 500-550 gsm. 10 cm square portions were cut out for testing, with the portions pressed on a lab Carver press to a density of 0.18-0.22 g/cc. Three portions were tested from each sample pad.

Table 4 lists wicking rates and absorption capacity properties of example absorbent cores made of 100% SBSK, and inclined plane wicking rates of example absorbent cores made of 30% SBSK and 70% SAP, using several SBSK samples produced in accordance with the present disclosure and having different coarseness and fiber length properties. The example absorbent cores produced from the SBSK samples (nominally numbered 1-5) are compared against a control in which the SBSK is a commercially available fluff pulp available from Weyerhaeuser Company under the designation CF416.

TABLE 4 Wicking Inclined plane rate, Absorp wicking rate, Sample Coarseness LWAFL no SAP Cap. 70% SAP Control 21.6 2.35 10.99 10.18 127.2 1 17.1 1.94 9.09 11.10 139.4 2 17.8 1.99 8.72 11.35 139.4 3 18.0 2.08 7.62 11.31 138.0 4 20.0 2.52 9.49 12.00 137.0 5 25.2 2.99 10.60 12.10 124.0

FIG. 2 provides a representative plot of wicking rates of the example 100% SBSK pulp cores listed in Table 4, and demonstrates a correlation between higher coarseness and higher wicking in the absence of SAP. FIG. 3, however, a representative plot of inclined plane wicking rates of the sample composite pulp cores listed in Table 4, demonstrates the unexpected (and unexpectedly strong) correlation between lower coarseness fibers and higher inclined plane wicking in the presence of a large proportion of SAP.

Although not wishing to be bound by theory, one possible explanation for this unexpected correlation may be related to interaction between individual fibers and SAP particles in a composite SAP and fiber structure. Lower coarseness generally indicates a higher fiber population per gram. A higher fiber population may in turn result in more fluid pathways within the fiber structure, by obstructing SAP particles from gel-locking with adjacent SAP particles as they swell as fluid is acquired by the structure.

Thus, a coarseness range for SBSK pulp sheets made in accordance with the present disclosure suitable to exhibit the desired inclined plane wicking rate in the presence of a large proportion of SAP appears to be about 16 to about 20 mg/100 m, over a LWAFL range of about 1.6 to about 2.7 mm.

Based on the aforementioned observed results, the unexpected increase in inclined plane wicking rate seen in absorbent cores in which the pulp component is made up of low coarseness SBSK is predicted to be preserved even when the low coarseness SBSK is blended with up to about 25% NBSK, and in particular low coarseness NBSK (e.g., of a coarseness of from about 13 to about 18 mg/100 m, and of a fiber length of from about 2.0 to about 2.5 mm), such as in an absorbent core with high levels of SAP. Accordingly, a pulp sheet in accordance with the present disclosure includes a blend of low coarseness (i.e., about 16-20 mg/100 m) Southern pine fibers with NBSK. The NBSK in said pulp sheets has a coarseness of about 13-18 mg/100 m and a fiber length of about 2.0-2.5 mm. Such a “blended” pulp sheet may be produced at a basis weight and/or density appropriate for applications such as conversion into absorbent products, such as a basis weight range of 500-900 gsm and/or a density of 0.4-0.8 g/cc.

In addition to the unexpectedly increased inclined plane wicking rate properties provided by lower coarseness pulp sheets produced from Southern pine fibers in accordance with the present disclosure, such lower coarseness pulps were also found to be more easily amenable to densification, such as when incorporated into an absorbent core for inclusion in absorbent products. Densification can result in thinner absorbent cores, which in turn may result in less bulky absorbent products, which are both aesthetically preferable to more bulky alternatives, and may also result in reduced shipping and storage costs, decreased shelf space, and so forth. However, densification is often limited by SAP, which can be damaged as a result of the application of pressure to a composite structure in which it is placed. SAP particles often consist of a highly-crosslinked polymeric shell encapsulating a less crosslinked polymeric core. This structure helps to prevent gel-locking with other SAP particles. If SAP particles are damaged, such as by compromising the outer shell, the fluid acquisition properties of the absorbent core may in turn be compromised, reducing the utility of such absorbent products.

FIG. 4 is a representative plot of density, which relates to thickness, against pressing pressure on a Carver press, for example composite absorbent cores containing SBSK fibers of varying coarseness. The absorbent cores were prepared as above, at a ratio of 30% SBSK to 70% SAP. Lower coarseness fibers result in greater density at the same applied pressure, or in other words, require less pressure to achieve the same density, as compared to higher coarseness fibers.

This effect is even more pronounced with the addition of softening agents such as glycerin, as shown in FIG. 5, which is also a representative plot of density against pressing pressure. An SBSK control core (CF416, coarseness 21.6) is shown for comparison against an SBSK core with, and without, 3% glycerin.

Although the present invention has been shown and described with reference to the foregoing operational principles and illustrated examples and embodiments, it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims. 

1-20. (canceled)
 21. A pulp sheet comprising Southern pine fibers, wherein a TAPPI T-205 tissue-grade handsheet having a target basis weight of 20 grams per square meter (gsm) produced therefrom exhibits a tensile stiffness according to TAPPI T-494, normalized to a tensile index of 1, of 0.040 or below.
 22. The pulp sheet of claim 21, wherein the Southern pine fibers have a fiber length to coarseness ratio of at least 0.11, wherein fiber length (LWAFL) is in mm, and coarseness is in mg/100 m.
 23. The pulp sheet of claim 22, wherein the length to coarseness ratio of the Southern pine fibers is at one or more of a coarseness of from about 16 to about 25 mg/100 m, and a fiber length of from about 1.6 to about 3.0 mm.
 24. The pulp sheet of claim 22, wherein the length to coarseness ratio of the fibers is at a coarseness of from about 19 to about 20 mg/100 m and a fiber length of from about 2.5 to about 2.7 mm.
 25. The pulp sheet of claim 22, wherein the Southern pine fibers have a fiber length to coarseness ratio of at least 0.12.
 26. The pulp sheet of claim 21, wherein the Southern pine fibers are refined.
 27. The pulp sheet of claim 26, wherein the TAPPI T-205 tissue grade handsheet exhibits a tensile stiffness according to TAPPI T-494, normalized to a tensile index of 1, of 0.037 or below.
 28. The pulp sheet of claim 21, wherein the content of Southern pine fibers is at least 20%.
 29. The pulp sheet of claim 28, also comprising hardwood fibers.
 30. The pulp sheet of claim 29, wherein the hardwood fibers comprise eucalyptus fibers.
 31. The pulp sheet of claim 30, comprising a blend of Southern pine fibers and eucalyptus fibers in a 1:2 ratio.
 32. The pulp sheet of claim 28, wherein the content of Southern pine fibers is at least 30%.
 33. The pulp sheet of claim 21, wherein the Southern pine fibers are bleached Southern pine pulp fibers.
 34. The pulp sheet of claim 21, wherein the Southern pine fibers are kraft Southern pine pulp fibers.
 35. The pulp sheet of claim 21, wherein the Southern pine fibers are SBSK pulp fibers.
 36. The pulp sheet of claim 21, wherein the Southern pine fibers are one or more of slash pine (Pinus elliottii), longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), loblolly pine (Pinus taeda), Virginia pine (Pinus virginiana), eastern white pine (Pinus strobus), pitch pine (Pinus rigida), sand pine (Pinus clausa), pond pine (Pinus serotina), and Table Mountain pine (Pinus pungens) fibers.
 37. The pulp sheet of claim 21, wherein a majority of the Southern pine fibers are loblolly pine (Pinus taeda) fibers.
 38. A composite pad comprising a matrix of pulp fibers and superabsorbent particles (SAP) distributed throughout the matrix of pulp fibers, wherein SAP constitutes at least 50% of the weight of the composite pad, and wherein Southern pine fibers having a coarseness of from about 16 to about 20 mg/100 m constitute at least 75% of the weight of the pulp fibers.
 39. The composite pad of claim 38, wherein the Southern pine fibers have a fiber length of from about 1.6 to about 2.7 mm.
 40. An absorbent core that incorporates the composite pad of claim
 38. 